1. Technical Field
The present disclosure relates to an antenna impedance modulation method, with application to contactless integrated circuits and, more particularly, to contactless integrated circuits of the passive type electrically powered by signals supplied by an antenna circuit.
2. Description of the Related Art
Contactless integrated circuits or RFID integrated circuits (Radio Frequency Identification) are used in various applications like the manufacture of electronic tags and contactless chip cards, for example electronic wallets, access control cards, transport cards, etc.
The disclosure more particularly relates to UHF contactless integrated circuits, provided to operate in the presence of an UHF electric field oscillating at a frequency of several hundreds MHz, generally ranging from 800 MHz to 100 GHz.
FIG. 1 schematically shows a contactless integrated circuit IC1 of the UHF type. The circuit IC1 includes an antenna circuit ACT, a primary charge pump PMP, a modulation circuit MCT and a demodulation circuit DCT, together forming a contactless communication interface. The integrated circuit also comprises a control unit CTU and a non-volatile memory MEM. The memory is for example an EEPROM memory (electrically erasable and programmable), allowing the integrated circuit to memorise transaction and identification data. The control unit CTU controls the access to the memory by executing commands for reading or writing in the memory.
The antenna circuit ACT includes two conductors W1, W2 forming a dipole. In the presence of an electric field E emitted by a reader RD1 schematically shown in the figure, antenna signals S1, S2 appear on the conductors W1, W2. These antenna signals S1, S2 are alternating signals of low amplitude, a few tenths of Volts only, and are in phase opposition.
The primary charge pump PMP is driven by the signals S1, S2, used as pump signals, and supplies a continuous voltage Vcc. The voltage Vcc is typically about one Volt to a few Volts, for example 1.8 V, and ensures the power supply of the integrated circuit if it is completely passive (that is without autonomous power supply, like a battery).
The circuit MCT receives from the control unit CTU data DTx to be sent via the antenna circuit, and modulates the impedance of the antenna circuit ACT according to these data. To that end, the circuit MCT applies to the charge pump PMP a modulation signal Sm(DTx) which contains the data DTx in coded form. The signal Sm(DTx) has a non-active value by default, for example 0, and, during the modulation periods, has an active value, for example 1, which has the effect of short-circuiting the charge pump.
When the signal Sm(DTx) is inactive, the antenna circuit ACT absorbs all the incident power Pi emitted by the reader RD1 and picked up by the antenna circuit ACT which impedance is adapted to that purpose. When the signal Sm(DTx) is 1, the short-circuit of the charge pump causes a modulation of the antenna circuit impedance and consequently a modulation of its reflection coefficient. The antenna circuit is then detuned and sends a reflected wave of power Pr. The reflected wave is received by the reader RD1 on its own antenna circuit, which outputs a modulated signal that is the image of the signal Sm(DTx). The reader extracts the modulated signal from its antenna circuit, by means of adapted filters, and deduces the data DTx, after demodulation and decoding. This technique of passive data transmission is generally called “backscattering”.
FIG. 2 shows the standard structure of the charge pump PMP and also shows a modulation switch SW1 controlled by the signal Sm(DTx) and arranged for short-circuiting the charge pump.
The charge pump PMP comprises three pump stages in series. Each stage comprises two diodes and two capacitors, the latter being connected to the conductors W1, W2 of the antenna circuit to receive the signals S1, S2. The modulation switch SW1 is arranged in parallel with the output of the last stage of the charge pump. The switch is ON (conducting) when Sm(DTx)=1 and is OFF (not conducting) when Sm(DTx)=0.
When the switch SW1 is ON, the output of the charge pump is short-circuited and the voltage Vcc is no longer produced. In order to avoid a total break of voltage Vcc supply, a hold capacitor Ch is added at the output of the charge pump. The capacitor Ch is linked to the output of the charge pump through an inverse-mounted isolation diode Di. Thus, when the switch SW1 short-circuits the output of the charge pump, the diode Di blocks itself and the capacitor Ch alone holds the voltage Vcc above a critical threshold under which the integrated circuit stops operating.
An auxiliary switch SW2, driven by a signal/Sm(DTx) supplied by an inverting gate IG1, is arranged in parallel with the diode Di. When the switch SW1 is OFF, the switch SW2 is ON and the diode Di is short-circuited. Thus, the capacitor Ch is charged at the voltage Vcc without loss of voltage at the terminals of the diode Di.
This method for modulating the impedance of the antenna circuit ACT, although essential for sending data by backscattering, has the drawback of completely neutralizing the production of continuous voltage Vcc by the charge pump. Thus, despite the provision of the hold capacitor Ch, the voltage Vcc rapidly decreases when the integrated circuit sends data. The periods of data emission are thus critical periods as far as energy reception is concerned, and define the maximum communication distance with the reader RD1.